(a) Field of the Invention
The present invention relates to lithium manganese complex oxide with a spinel structure that is used as a cathode active material of a lithium or lithium ion secondary battery. More particularly, the present invention relates to a method for preparing lithium manganese complex oxide having improved cyclic performance at a temperature range of room temperature or more, and a lithium or lithium ion secondary battery using the oxide prepared according to the method as a cathode active material.
(b) Description of the Related Art
Lithium manganese complex oxide with a spinel structure is one of the active materials largely studied recently, because it has advantages in terms of its stability in lithium secondary battery of 4 V (volt) and cost over other active materials.
However, when lithium manganese complex oxide is used as an active material for a lithium secondary battery, the capacity of the battery decreases as charge-discharge cycle proceeds, and the capacity seriously decreases at a high temperature of 40° C. or more and thus the practical applications thereof are under many restrictions. The reasons for such decrease in capacity are various, but representatively, Mn3+ in lithium manganese complex oxide decomposes to the forms of Mn2+ and Mn4+ by disproportionation reaction as shown in the following Equation 1, and Mn2+ is dissolved into an electrolyte.2Mn3+→Mn4++Mn2+  [Equation 1]
Accordingly, the foci of studies concentrate on decreasing Mn3+ existing in a spinel compound even if this involves decreasing capacity, in order to improve life characteristics. Representative methods include a stabilization of the spinel structure by doping different kinds of metal, and coating a different kind of compound on a surface of the spinel particle, etc. However, although said methods can improve life characteristics, they cause a decrease in initial capacity of the spinel compound because the capacity of the spinel compound is proportional to the concentration of Mn3+.
U.S. Pat. No. 5,759,717 stated that coating Li2CO3, Na2CO3, K2CO3 etc. on lithium manganese complex oxide resulted in both an improvement in room temperature life characteristics of a spinel compound and a low degree of self-discharge even when stored in a charged condition at 60° C. However, said method did not mention high temperature life characteristics.
A method of inhibiting oxidation/reduction on the surface of spinel using additives such as MgO, CaO, BaO and SiO2 for inhibiting dissolution of Mn2+ is known. According to said method, H2O that is absorbed on the surface of spinel or exists in the form of impurities in an electrolyte and promotes the dissolution of Mn2+ is removed thereby inhibiting the dissolution of manganese. However, since added oxides have a large volume per weight, when a cathode electrode plate is prepared using spinel prepared according to said method, the amounts of active material (spinel) per volume decreases and consequently the capacity of the cathode electrode decreases.
A method of decreasing the specific surface area of a spinel compound is known. However, said method requires a long heat treatment because the size of particles should increase in order to decrease the specific surface area. In addition, when the particle is large, the diffusion distance of lithium in spinel increases thereby decreasing the C-rate (charge rate) in the charge/discharge process.
A method of coating an amorphous phase having ion conductivity for lithium on the surface of a spinel particle is also known. However, said method cannot prevent a decrease in capacity and cannot improve high temperature cyclic performance.